The present invention relates to a small-sized reciprocating gas compressor and particularly to an oilless gas compressor and more particularly to an oilless gas compressor for use at cryogenic temperatures.
Generally, reciprocating compressors require lubricating oil to be used between the piston and the cylinder. In cases where such compressor is used as a gas compressor, e.g., as a nitrogen gas compressor in a nitrogen liquefying system, it is required that no lubricating oil be used between the piston and the cylinder, since migration of lubricating oil to the nitrogen would result in the icing of oil, which is undesirable from the standpoint of production of cryogenic temperature. Such a gas compressor is called an oilless gas compressor.
Various types of oilless gas compressors are known. These include the use of a fluid lubricant other than lubricating oil in the cylinder or the use of a special material which is lubricative for the inner slide area of the cylinder without using liquid lubricants. It is also known to use a diaphragm or bellows for indirect intake and exhaust of gas. Another approach includes the use of a rod packing being in contact with the piston rod, which is reciprocating through the guide piston or the use of some other form of oil creep-up preventing device, thereby preventing oil from moving along the piston rod to enter the cylinder.
However, these known types of oilless gas compressors can hardly be practical as cryogenic temperature producing gas compressors, particularly as small-sized ones.